package LuoGu;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class P2678 {
    static int M;
    static int L;
    static int N;
    static int[] a;

    public static boolean ifOK(int i) {
        int now = 0;
        int index = 0;
        int tot = 0;
        while (index < N + 1) {
            index++;
            if (tot > M) {
                return false;
            }
            if (a[index] - a[now] < i) {
                tot++;
            } else {
                now = index;
            }
        }
        if (tot > M) {
            return false;
        }
        return true;
    }

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String tmp = br.readLine();
        L = Integer.parseInt(tmp.split(" ")[0]);
        N = Integer.parseInt(tmp.split(" ")[1]);
        M = Integer.parseInt(tmp.split(" ")[2]);
        a = new int[N + 2];
        a[0] = 0;
        for (int i = 1; i <= N; i++) {
            tmp = br.readLine();
            tmp = tmp.replaceAll(" ","");
            a[i] = Integer.parseInt(tmp);
        }
        a[N + 1] = L;
        int left = 1;
        int right = L;
        int ans = 0;
        while (left <= right) {
            int mid = (left + right) >>> 1;
            if(ifOK(mid)){
                ans = mid;
                left = mid+1;
            }else{
                right = mid-1;
            }
        }
        System.out.println(ans);

    }
}
